Bullying or ganging up on a player should be a reportable offense.
This is the one thing that I experience more than anything else in the game. Especially when I'm with a premade I'm the odd one out and I am harassed and bullied until I mute everyone. When I lose lane I already feel bad enough. I don't need people telling others to report me for trolling when I'm 4/5/4.
I think this behavior should be punished heavily. The whole "report X" mentality is pretty toxic.